1. Field of the Invention
This invention relates to methods for using the transfer of materials and layers to prepare optically useful elements, such as liquid crystal display (LCD) panels. In particular, this invention relates to a method for assembling a planarization and/or indium-tin-oxide layer in an optical element such as a color filter element for use in liquid crystal display devices. An adhesive layer comprising a crosslinkable adhesive layer which may be a pressure-sensitive or low temperature softening thermal adhesive layer is used in the transfer process, this adhesive layer also having the capability of performing as a planarization layer in the final article.
2. Background of the Art
Liquid crystal display devices have competed with phosphorescent display systems and have created their own unique markets. Liquid crystal displays are commonly found in digital imaging systems such as watches, calculators, computer displays, audio/video equipment, and other electric appliances to provide the readable image. Liquid crystal displays provide excellent properties which enable them to compete with other display technologies, and among these properties are low power requirements, small volume needs, little heat generation, and acceptable resolution. Liquid crystal arrays have had to provide color display capability to satisfy the wide range of desirable markets for their use, and coloration has been provided by associating color filters (e.g., flat panel color filters or displays) with the liquid crystals. These color filters transmit only selected and limited wavelengths of radiation (by absorbing the undesired wavelengths), thus allowing light of specified color to pass from the liquid crystal. To provide the best results, color filters are adhered to the surface of liquid crystal display devices as opposed to being merely suspended over the display area of the liquid crystal device. The entire liquid crystal display system usually comprises more than one or two layers. Including the color filter device, the system will generally comprise a first substrate (e.g., glass plate), conductive layer, alignment layer, liquid crystal layer, second alignment layer, conductive layer, planarization layer, color array, black matrix, and second substrate (e.g., second glass plate).
As shown in U.S. Pat. No. 5,166,126, a binder layer between the conductive layer and the second alignment layer may be present. The physical process of associating of these layers into a single article can be complex. The deposition of the conductive layer, when made of indium-tin-oxide materials, can be particularly difficult because of the temperatures generated from necessary heat treatment of the layer during annealing.
U.S. Pat. No. 5,166,126 uses an adhesive thermal buffer layer between the conductive layer and the liquid crystal display device to reduce the temperature impact of the deposition and treatment of the conductive layer.
U.S. patent application Ser. No. 08/273,419, filed Jul. 11, 1994 and U.S. Pat. No. 5,521,035 disclose the use of thermal mass and thermal dye transfer processes to manufacture color filter display panels by the transfer of colorant (dye and/or pigment, with or without a binder) onto a panel to lay over the liquid crystal display.
A series of patents (U.S. Pat. Nos. 4,965,242, 4,962,081, 4,975,410, 4,923,860, 5,073,534, and 5,166,126) have been assigned to Kodak which disclose the use of thermal dye diffusion transfer to make filter elements and color filter constructions. U.S. Pat. Nos. 4,965,242 and 5,073,534 teach the use of high Tg polycarbonate and polyester receiving layers to accept the thermally transferred dye. With both receiving layers, a vaporous solvent treatment is required to drive the dye into the receiving layer.
For a color filter element to be useful as a component of certain types of optical display devices, including for example liquid crystal and phosphorescent displays, the color filter array is preferably coated with a planarizing layer. This planarizing layer (which, amongst other objectives, levels or smooths the surface of the color filter element for acceptable placement of subsequent layers) is usually followed by a coating of a transparent conductor, commonly indium tin oxide (ITO). The conductive layer may or may not be patterned, and is preferably a continuous layer. Finally, an alignment layer, typically a resin such as a polyimide, is applied onto the conductive layer. The alignment layer is typically patterned (e.g., by brushing) to control the alignment of the liquid crystal material in the functioning display.
The substrate may be any substance upon which a color filter or the like is desired to be formed. Preferably the substrate is a transparent (at least light transmissive) substrate, such as glass, polymer film, and the like. When glass is the substrate, the use of glass treated with silane (e.g., 3-aminopropyltriethoxysilane) or titanate coupling agents may be useful to increase adhesion of the colorant layer. Possible substrates include glass, polyester base (e.g., polyethylene terephthalate, polyethylene naphthalate), polycarbonate resins, polyolefin resins, polyvinyl resins (e.g., polyvinyl chloride, polyvinylidene chloride, polyvinyl acetals, etc.), cellulose ester bases (e.g., cellulose triacetate, cellulose acetate), and other conventional polymeric films used as supports in various imaging arts. Transparent polymeric film base of 2 to 100 mils (e.g., 0.05 to 2.54 mm) is preferred. If the substrate is a polymeric film, it is preferred that the film be non-birefringent so as not to interfere with the operation of the display in which it may be integrated. Preferred examples of non-birefringent substrates are polyesters that are solvent cast. Typical examples of these are those derived from polymers consisting or consisting essentially of repeating, interpolymerized units derived from 9,9-bis-(4-hydroxyphenyl)-fluorene and isophthalic acid, terephthalic acid or mixtures thereof, the polymer being sufficiently low in oligomer (i.e., chemical species having molecular weights of about 8000 or less) content to allow formation of a uniform film. This polymer has been disclosed as one component in a thermal transfer receiving element in U.S. Pat. No. 5,318,938. Another class of non-birefringent substrates are amorphous polyolefins (e.g., Zeonex.TM. from Nippon Zeon Co., Ltd.). The most preferred substrate is glass. It is preferred that this glass be 0.5 to 2.0 mm thick. Especially preferred thicknesses are between 0.5 and 1.5 mm, such as 0.7 and 1.1 mm.
The colors to be deposited on the substrate sheet for the color filter element may comprise any color material which can be deposited with adherence to the substrate. In a preferred embodiment, the colorant is in a suitable binder system and is solvent coated.
When pigments are used as the color material, they are preferably transparent. Examples of transparent pigments that can be used in this invention include Sun RS Magenta 234-0077.TM., Hoechst GS Yellow GG11-1200.TM., Sun GS Cyan 249-0592.TM., Sun RS Cyan 248-0615.TM., Ciba-Geigy BS Magenta RT-333D.TM., Ciba-Geigy Microlith Yellow 3G-WA.TM., Ciba-Geigy Microlith Yellow 2R-WA.TM., Ciba-Geigy Microlith Blue YG-WA.TM., Ciba-Geigy Microlith Black C-WA.TM., Ciba-Geigy Microlith Violet RL-WA.TM., Ciba-Geigy Microlith Red RBS-WA.TM., any of the Heucotech Aquis II.TM. series, any of the Heucosperse Aquis III.TM. series, and the like. A preferred method of inserting or depositing the colors on the matrix is by laser induced mass transfer, including a "melt stick" and/or an ablative transfer process in which a donor sheet having the colors thereon is used to transfer colors onto the substrate. "Ablative transfer" refers to a process in which a medium is ablated in thermal imaging processes by the action of a thermal source, through the rapid removal of material from the surface, but without sublimation of the material. Such donor sheets are known in the art for direct image forming.
Thermal mass transfer of colorant onto a support is a significant improvement over dye (e.g., sublimation) transfer in forming color filter elements. The colors are more durable with respect to both abrasion and color fading (when pigments are used). The transferred mass material carries its own binder and can be applied to a greater range of substrate materials. No post-treatment of the transferred mass colorant is needed as may be required for dye transferred materials used in color filters of the prior art (e.g., U.S. Pat. Nos. 4,965,242 and 5,073,534).
The colors used to form the color filter are generally the primary additive colors, i.e., red, green, and blue. Each of these primary colors preferably has high color purity and transmittance, and, when combined, an appropriate white balance. The color filters preferably have spectral characteristics of red, green, and blue that show chromaticity close to the National Television Standards Committee (NTSC) standard colors indicated by the Commission International de 1'Eclairage (CIE) chromaticity diagram. Although red, green, and blue are the most common colors for the filters, other color combinations may be used for specialty applications. In some cases, the repeat sequence in a row is red:green:blue. For other applications the repeat sequence in a row is red:green:green:blue.
A general description of color filters for liquid crystal displays is given in C. C. O Mara, Liquid Crystal Flat Panel Display: Manufacturing Science and Technology, Van Norstrand Reinhold, 1993 p. 70. Several fabrication methods are disclosed. The most common method for preparing color filters is using photolithographic techniques. One photolithographic process is detailed in an article entitled "Color Filters from Dyed Polyimides" W. J. Latham and D. W. Hawley, Solid State Technology, May 1988. This paper shows the complex, multi-step nature of the photolithographic process.
The shapes of the color elements may be simple geometric objects such as rectangles, squares or triangles. Alternatively, for some configurations of color filters, the color elements may be created as stripes. Another common configuration for a color filter array is when the color elements in one row are displaced by one element in the second row and by two elements in the third row such that the color elements are diagonally aligned.
The dimensions of the discrete elements can range from 5-1000 microns.
More typically the dimensions are on the order of 50-300 microns. These dimensions are easily produced by photolithographic and laser imaging techniques.
There are many transparent electrically conducting laminate elements described in the literature. A Kokai Patent Application No. HEI 51993!-177757 discloses a light shield film comprising a laminate construction containing the following layers in the order listed: 1) a transparent film base, a heating layer which in the examples is an ITO layer, a moisture proof SiO.sub.2 layer, a sol-gel layer which undergoes a transition from a transparent to scattering when heated, and another transparent film. Kokai Patent Application No. HEI 21990!-213006 (Nitto Electric Corp.) teaches the construction of transparent, conductive laminates. These films are prepared by depositing a conductive film on one side of a transparent film substrate and a dielectric layer onto the conductive layer; the other side of the substrate layer is then coated with a transparent adhesive. The dielectric layer serves the dual purpose of protecting the conductive layer from scratching and also reduces reflection from the surface. The adhesives discussed are acrylics, for example a copolymer of butylacrylate, acrylic acid and vinyl acetate which has been crosslinked with an isocynate type crosslinking agent. The final construction is prepared by bonding the adhesive layer to a thicker transparent support layer such as a polyester. The adhesive properties are chosen to provide a cushioning effect. Kokai Patent Application No. HEI 11989!-149003 discloses laminates for VDT filters. The laminates are prepared by bonding a number of layers together using a crosslinkable ethylene copolymer adhesive. The ethylene copolymers include ethylene-vinylacetate, ethylene-methacrylate copolymer, ethylene methacrylic acid copolymer, ethylene-ethylmethacrylate copolymer, ethylene methylacrylate copolymer, metal ion crosslinked ethylene-ethylmethacrylic acid copolymer, partially saponified ethylene-vinyl acetate copolymer, and carboxylated ethylene vinyl acetate copolymer. The crosslinking of these polymers can be accomplished using heat or light. One method detailed in the patent application is the use of these polymers with an initiator system consisting of organic peroxides. The multifunctional acrylates, or allyl oligomers can also be used in this invention. Adhesion promoters such as silane coupling agent containing epoxy groups or acrylates may also be present. For light induced crosslinkable systems a sensitizer is provided. In order to prevent blocking it was necessary to emboss the adhesive surface. Another Kokai Patent Application No. SHO 621987!-180329 involves a construction discloses the use of a laminate construction comprising a transparent conductive layer, polymeric substrate layer, an optional primer layer, a ethylene-vinyl alcohol containing layer, and a thermoset containing layer. The purpose of the thermoset layer is to protect the EVA layer in subsequent etching steps performed on the ITO layer. Other laminate constructions are described in Kokai Patent Application No. Sho 601985!-222241 wherein a transparent substrate layer is coated with a curable composition to improve the hardness of the laminate construction. In all the examples the curable layer is completely cured and as such is not suitable for laminating to another optical element without application of an adhesive. Other less relevant laminate constructions are described in Kokai Patent Application No. Sho 611986!-32749 and Kokai Patent Application No. Sho 621987!-144943.
In Kokai Patent Application No. Hei 61994!-91223 the use of a release material is disclosed in a method for smoothing polymeric films to prevent the formation of irregular patterns. The specific example cited is the formation of a planarized surface on a color filter element. The process involves bonding a resin film on a base or substrate layer and then depositing a release film (smoothing layer) on top of the resin layer by applying pressure and heat to smooth the surface. The release film is then removed. The release layer is comprised of a base layer, a conductive layer and an adhesive layer (on the conductive layer). The base layer and conductive layer of the smoothing film are stripped from the color filter element. This differs from the inventions described herein in which the conductive layer remains with the color filter element. Also the conductive layer need not be transparent in their invention as carbon black is listed as a possible conductive layer.
There is also a Kodak patent that discuss laminate ITO layers with an associated color filter, U.S. Pat. No. 4,965,242. In this construction, a temporary support layer is laminated to a polymeric alignment layer on top of a transparent conducting layer. In this case, the polymeric alignment layers are polyimides, polyvinylalcohols and methyl cellulose. The temporary support layer is a Kaptan resin (a polymer of the diimide of pyromellitic acid and 4,4'-oxydianiline).